Method and apparatus for transmitting signals

ABSTRACT

A storage unit stores a preamble signal defined in a legacy system and a preamble signal defined in a MIMO system. A monitoring unit in a transmitting apparatus monitors the existence of any communication apparatus which is not compatible with the MIMO system but accepts the legacy system. A channel characteristics acquiring unit derives the characteristics of a radio channel between the transmitting apparatus and a receiving apparatus. A selector selects a packet format based on a monitoring result obtained by the monitoring unit. The selector also selects where to place LTS, based on the characteristics of wireless channel derived by the channel characteristics acquiring unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the signal transmission technologies,and it particularly relates to method and apparatus for transmittingsignals in the packet format.

2. Description of the Related Art

In wireless communication, it is generally desired that the limitedfrequency resources be used effectively. Adaptive array antennatechnology is one of the technologies that realize the effectiveutilization of frequency resources. In adaptive array antennatechnology, the amplitude and phase of signals transmitted from andreceived by a plurality of antennas, respectively, are so controlled asto form a directional pattern of the antenna. In other words, theapparatuses provided with adaptive array antennas change respectivelythe amplitudes and phases of signals received by a plurality ofantennas, add up the thus changed received signals, and receive signalsequivalent to the signals received by the antenna having the directionalpattern corresponding to the variation in said amplitude and phase(hereinafter referred to as “weight”). And the signals are transmittedin a directional pattern of the antenna corresponding to the weight.

One example of processings to compute the weights in adaptive arrayantenna technology is a processing based on the MMSE (Minimum MeanSquare Error) method. In the MMSE method, the Wiener solution is knownto be the condition for an optimum weight value. Also known is arecurrence formula whose amount of calculation is smaller than thatrequired to directly solve the Wiener solution. For such recurrenceformula, adaptive algorithms, such as RLS (Recursive Least Squares)algorithm and LMS (Least Mean Squares) algorithm, are used. On the otherhand, for the purpose of realizing a higher data transmission rate andimproving the transmission quality, there is a case where data aresubjected to multi-carrier modulation and the multi-carrier signals aretransmitted (See Reference (1) in the following Related Art List, forinstance).

Related Art List

-   (1) Japanese Patent Application Laid-Open No. Hei10-210099.

There is a MIMO (Multiple Input Multiple Output) system as one oftechnologies by which to raise the transmission rate of data using theadaptive array antenna technology. A transmitting apparatus and areceiving apparatus in the MIMO system are each equipped with aplurality of antennas, and a channel suitable for each of the antennasis set. That is, the channel of up to the maximum number of antennas isset for the communication between the transmitting apparatus and thereceiving apparatus, so as to improve the data transmission rate.Moreover, combining the MIMO system with a technique of transmittingmulti-carrier signals results in a higher data transmission rate. On theother hand, the transmitted signals generally contain preambles that areknown signals in order for the signals transmitted from the transmittingapparatus to be accurately received. In general, a preamble signal isspecified by a fixed pattern. Nevertheless, if the pattern of a preamblesignal varies taking into account the characteristics of radio channeland the packet utilization efficiency, it is possible to realize awireless communication system which is flexible in terms of thecharacteristics of radio channel and the like.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoingcircumstances and problems, and an object thereof is to provide methodand apparatus by which to vary the format of preamble signal.

In order to solve the above problems, a transmitting apparatus accordingto a preferred mode of carrying out the present invention comprises: astorage unit which stores a first known signal defined in a firstwireless communication system and a second known signal defined in asecond wireless communication system which differs from the firstwireless communication system; a selector which selects either a firstpacket format in which the second known signal is placed in a frontportion thereof or a second packet format in which the first knownsignal is further placed before the second known signal; and atransmitter which transmits signals in the packet format selected by theselector.

According to this mode of carrying out the present invention, thepresence or absence of the first preamble signal is switched, so thatthe improvement in the compatibility with the first wirelesscommunication system and the packet utilization efficiency in the secondwireless communication system can be selected.

Another preferred mode of carrying out the present invention relatesalso to a transmitting apparatus. This apparatus comprises: a storageunit which stores a first known signal defined in a first wirelesscommunication system which is to transmit signals using a plurality ofcarriers and a second known signal defined in a second wirelesscommunication system which is to transmit signals from a plurality ofantennas in parallel, using the same number of carriers to transmit thesignals as in the first wireless communication system; a selector whichselects either a first packet format in which the second known signal isplaced in a front portion thereof, or a second packet format in whichthe first known signal is further placed before the second known signal;and a transmitter which transmits signals in the packet format selectedby the selector.

According to this mode of carrying out the present invention, thepresence or absence of the first preamble signal is switched, so thatthe improvement in the compatibility with the first wirelesscommunication system and the packet utilization efficiency in the secondwireless communication system can be selected.

The second known signal stored in the storage unit may be defined in aplurality of kinds in accordance with the number of antennas which areto transmit signals in the second wireless communication system. Sincethe pattern of the second known signal is changed in accordance with thenumber of antennas, the communication quality can be improved.

If the packet format in which the second known signal is placed in afront portion thereof is selected and the number of antennas to transmitsignals is one, the selector may assign one of the second known signalsin which the plurality of kinds are defined. Even if the number ofantennas becomes one from a plural number, the second known signalcorresponding to one of a plurality of antennas is used. Thus, theswitching to the first wireless communication system is no longernecessary.

When the second packet format in which the first known signal is furtherplaced before the second known signal is selected, the selector mayassign information indicating that the second known signal is placed,between the first known signal and the second known signal. Since theinformation indicating that the second known signal has been placedafter the first known signal is inserted, the content of such asubsequent signal can be conveyed to a communication apparatus of thefirst wireless communication system.

The transmitting apparatus may further comprise a monitoring unit whichmonitors the presence of a communication apparatus which is notcompatible with the second wireless communication system and iscompatible with the first wireless communication system, wherein theselector may select a packet format based on a monitoring resultobtained from the monitoring unit. The switching between the presenceand the absence of the first known signal is done based on whether anyterminal apparatus of the first wireless communication system exits ornot. Hence, no adverse effect will be given on other communicationapparatus even if the switching is carried out.

Still another preferred mode of carrying out the present inventionrelates also to a transmitting apparatus. This apparatus comprises: atransmitter which transmits signals defined in a predetermined packetformat, in parallel from a plurality of antennas; storage unit whichstores a known signal to be placed in a front portion of a packetformat; and a selector which selects, at the time of placing a knownsignal in the front portion of a packet format, either a firstassignment in which the known signal is transmitted at the same timingfrom the plurality of antennas or a second assignment in which the knownsignal is transmitted at different timings from the plurality ofantennas.

According to this mode of carrying out the present invention, theassignment of a preamble signal to be transmitted from a plurality ofantennas is varied. Thus, the transmission quality of signals and thepacket utilization efficiency can be selected.

The transmitting apparatus may further comprise a derivation unit whichderives characteristics of a radio channel through which signals are tobe transmitted, wherein the selector may select the assignment of knownsignal based on the characteristics of a radio channel derived by thederivation unit. The structure of preamble signals to be transmittedfrom a plurality of antennas is varied based on the quality of a radiochannel, so that the structure of a preamble suitable for the radiochannel in use can be selected.

Still another preferred mode of carrying out the present inventionrelates to a transmitting method. This method is characterized in that afirst known signal defined in a first wireless communication systemwhich is to transmit signals using a plurality carriers is specified, asecond known signal defined in a second wireless communication systemwhich is to transmit signals in parallel from a plurality of antennasusing the same number of carries as the number of carries through whichto transmit the signals are specified, and the signals are transmittedby selecting either a first packet format in which the second knownsignal is placed in a front portion or a second packet format in whichthe first known signal is further placed before the second known signal.

Still another preferred mode of carrying out the present inventionrelates also to a transmitting method. This method comprises: storing afirst known signal defined in a first wireless communication system anda second known signal defined in a second wireless communication systemwhich differs from the first wireless communication system; selectingeither a first packet format in which the second known signal is placedin a front portion thereof or a second packet format in which the firstknown signal is further placed before the second known signal; andtransmitting signals in the packet format selected by the selecting.

Still another preferred mode of carrying out the present inventionrelates also to a transmitting method. This method comprises: storing afirst known signal defined in a first wireless communication systemwhich is to transmit signals using a plurality of carriers and a secondknown signal defined in a second wireless communication system which isto transmit signals from a plurality of antennas in parallel, using thesame number of carriers to transmit the signals as in the first wirelesscommunication system; selecting either a first packet format in whichthe second known signal is placed in a front portion thereof, or asecond packet format in which the first known signal is further placedbefore the second known signal; and transmitting signals in the packetformat selected by the selecting.

The second known signal stored in the storing may be defined in aplurality of kinds in accordance with the number of antennas which areto transmit signals in the second wireless communication system. If thepacket format in which the second known signal is placed in a frontportion thereof is selected and the number of antennas to transmitsignals is one, the selecting may assign one of the second known signalsin which the plurality of kinds are defined. When the second packetformat in which the first known signal is further placed before thesecond known signal is selected, selecting may assign informationindicating that the second known signal is placed, between the firstknown signal and the second known signal.

The transmitting method may further comprise monitoring the presence ofa communication apparatus which is not compatible with the secondwireless communication system and is compatible with the first wirelesscommunication system, wherein the selecting may select a packet formatbased on a monitoring result obtained in the monitoring. The secondsignal stored in the storing may have a plurality of portions whosesignal patterns differ from each other, and the selecting may selecteither a first assignment of the second known signal in which at leastone of the plurality of portions are transmitted respectively at thesame timing from a plurality of antennas or a second assignment of thesecond known signal in which at least one of the plurality of portionsare transmitted respectively at different timings from the plurality ofantennas. The method may further comprise deriving characteristics of aradio channel through which signals are to be transmitted, wherein theselecting may select the assignment of known signal based on thecharacteristics of a radio channel derived by the deriving.

Still another preferred mode of carrying out the present inventionrelates also to a transmitting method. This method is such that either afirst assignment in which a known signal is transmitted at the sametiming from a plurality of antennas or a second assignment in which theknown signal is transmitted at different timings from the plurality ofantennas is selected for the known signal to be placed in a frontportion of a packet format of signal to be transmitted in parallel fromthe plurality of antennas.

Still another preferred mode of carrying out the present inventionrelates also to a transmitting method. This method comprises:transmitting signals defined in a predetermined packet format, inparallel from a plurality of antennas; storing a known signal to beplaced in a front portion of a packet format; and selecting, at the timeof placing a known signal in the front portion of a packet format,either a first assignment in which the known signal is transmitted atthe same timing from the plurality of antennas or a second assignment inwhich the known signal is transmitted at different timings from theplurality of antennas. The method may further comprise derivingcharacteristics of a radio channel through which signals are to betransmitted, wherein the selecting may select the assignment of knownsignal based on the characteristics of a radio channel derived by thederiving.

It is to be noted that any arbitrary combination of the above-describedstructural components and expressions of the present invention changedamong a method, an apparatus, a system, a recording medium, a computerprogram and so forth are all effective as and encompassed by the presentembodiments.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be sub-combinationof these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a spectrum of a multi-carrier signal according to thepresent embodiment.

FIG. 2 illustrates a structure of a packet format according to thepresent embodiment.

FIG. 3 illustrates a concept of a communication system according to thepresent embodiment.

FIG. 4 illustrates a structure of the transmitting apparatus shown inFIG. 3.

FIG. 5 illustrates a structure of the control unit shown in FIG. 4.

FIGS. 6A and 6B illustrate packet formats selected by the selector shownin FIG. 5.

FIGS. 7A and 7B illustrate formats of LTS selected by the selector shownin FIG. 5.

FIG. 8 is a table showing a relationship, used when the selection isdone at the selector shown in FIG. 5, between the number of transmittingantennas and the patterns of STSs transmitted by the transmittingantennas.

FIG. 9 illustrates a structure of the receiving apparatus shown in FIG.3.

FIG. 10 illustrates a structure of the first radio unit shown in FIG. 9.

FIG. 11 illustrates a structure of the correlator shown in FIG. 10.

FIG. 12 illustrates a structure of the first processing unit shown inFIG. 9.

FIG. 13 is a flowchart showing a procedure for transmission processingin the transmitting apparatus shown in FIG. 3.

FIG. 14 is another flowchart showing a procedure for transmissionprocessing in the transmitting apparatus shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on the following embodimentswhich do not intend to limit the scope of the present invention butexemplify the invention. All of the features and the combinationsthereof described in the embodiments are not necessarily essential tothe invention.

Before describing the present invention in detail, an outline of thepresent invention will be described first. The present embodimentrelates to a MIMO system which comprises a transmitting apparatusequipped with a plurality of antennas and a receiving apparatus equippedwith a plurality of antennas. The MIMO system according to the presentembodiment transmits signals by multi-carriers, or more specificallyOFDM (Orthogonal Frequency Division Multiplexing) modulation scheme, andthe transmitted signals are defined and specified in the packet format.A preamble signal is placed at a front portion of a packet format. And areceiving apparatus, which has received a signal, carries out thesetting of AGC (Automatic Gain Control), the synchronization of timing,the recovery of carriers and the like, based on the preamble signal. Inthe MIMO system, independent signals are transmitted from a plurality ofantennas of a transmitting apparatus and a receiving apparatusdemodulates desired signals by separating received signals by adaptivearray signal processing.

On the other hand, there are cases where in the vicinity of atransmitting apparatus there exists a receiving apparatus which is notcompatible with a MIMO system (hereinafter, a system not compatible witha MIMO system will be referred to as “legacy system”). Although thelegacy system transmits signals by the OFDM modulation scheme the sameway as the MIMO system does, it differs from the MIMO system in that thesignals are transmitted by setting one channel between the transmittingapparatus and the receiving apparatus. Now, if a preamble signalcompatible with the MIMO system only is added, the signal redundancy inthe packet format in the MIMO system can be reduced. However, since thelegacy system cannot recognize such a preamble signal, there are caseswhere the arrival of signals cannot be recognized. This corresponds tothe carrier sense being not accurately executed if the legacy systemuses CSMA (Carrier Sense Multiple Access). Thus, since the legacy systemjudges in this case that the signals are not transmitted and, for thatreason, mistakenly transmits the signals by itself, the rate ofoccurrence of signal collision increases.

In contrast thereto, if a preamble signal compatible with the legacysystem is added before the preamble signal compatible with the MIMOsystem only, the legacy system can also recognize the preamble signal,thus making it hard to cause the above-described problem. Nevertheless,since the preambles compatible with both the two systems are added inthis case, the signal redundancy in the packet format in the MIMO systemincreases. Hence, a transmitting apparatus according to the presentembodiment is such that a preamble signal compatible with a legacysystem is added to the front portion of a packet format if a receivingapparatus compatible with the legacy system exists in the vicinity ofthe transmitting apparatus whereas the preamble signal compatible withthe legacy system is not added to the front portion of a packet formatif the receiving apparatus compatible with the legacy system does notexist in the vicinity of the transmitting apparatus. It is to be notedhere that the “front portion” represents a part of leading portion,located near the head of a packet format, which also includes the headthereof.

FIG. 1 illustrates a spectrum of a multi-carrier signal according to thepresent embodiment. This corresponds to a multi-carrier signaltransmitted from the legacy system and a multi-carrier signaltransmitted from one of a plurality of antennas in the MIMO system.Here, assume that the legacy system is a wireless LAN (Local AreaNetwork) conforming to the IEEE802.11a standard (hereinafter, a wirelessLAN system in compliance with IEEE802.11a standard will be referred toas “legacy system”, too). One of a plurality of carriers in an OFDMscheme is generally called a subcarrier. Herein, however, each of thesubcarriers is designated by a “subcarrier number”. As illustrated inFIG. 1, the IEEE802.11a standard defines 53 subcarriers, namely,subcarrier numbers “−26” to “26”. It is to be noted that the subcarriernumber “0” is set to null so as to reduce the effect of a direct currentcomponent in a baseband signal. Also, the respective subcarriers aremodulated by BPSK (Binary Phase Shift Keying), QSPK (Quadrature PhaseShit Keying), 16 QAM (Quadrature Amplitude Modulation) and 64 QAM.

FIG. 2 shows a structure of a packet format according to the presentembodiment. This corresponds to a traffic channel of a legacy system. Inthe OFDM modulation scheme, the total sum of the size of Fouriertransform and the number of symbols of a guard interval generallyconstitutes one unit. This “one unit” is called an OFDM symbol in thepresent embodiment. In the legacy system, the size of Fourier transformis 64 (hereinafter the points of one FFT (Fast Fourier Transform) willbe called “FFT point”) and the FFT point number of a guard interval is16, so that the OFDM symbol corresponds to 80 FFT points.

A packet signal is such that a “preamble” composed of “4 OFDM symbols”is placed in the leading portion of the packet signal and a “signal” of“1 OFDM symbol” and “data” of arbitrary length in this order are placedsubsequent to the “preamble”. The “preamble” is a known signal used forthe setting of AGC, timing synchronization and carrier recovery and thelike in a receiving apparatus. The “signal” is a control signal whereasthe “data” is information to be transmitted from a transmittingapparatus to a receiving apparatus. As shown in FIG. 2, the “preamble”composed of “4 OFDM symbols” are separated into “STS (Short TrainingSequence)” composed of “2 OFDM symbols” and “LTS (Long TrainingSequence)” composed of “2 OFDM symbols”. STS is constituted by tensignal units “t₁” to “t₁₀” and each signal unit, such as “t₁”, is equalto 16 FFT points. Though STS is 16 FFT points in time-domain unit asdescribed above, it uses, in frequency domain, 12 subcarriers among 53subcarriers as shown in FIG. 1. STS is used particularly for the settingof AGC and the timing synchronization. LTS, on the other hand, isconstituted by two signal units “T₁” and “T₂” and a guard interval “GI2”which is twice as long as “t₁”. And one signal unit such as “T₁” is 64FFT points whereas “GI2” is 32 FFT points. LTS is used particularly forthe carrier recovery.

A signal in the frequency domain as shown in FIG. 1 is expressed byS_(−26, 26), where the subscript indicates the subcarrier number. Usingsuch notation as this, STS of legacy system is expressed as in thefollowing Equation (1).S _(−26, 26) =sqrt(13/6){0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,0,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0}  (1)where “1+j” denotes the signal point of STS after a QPSK modulation.

FIG. 3 illustrates a concept of a communication system 100 according tothe present embodiment. The communication system 100 includes atransmitting apparatus 10 and a receiving apparatus 12. The transmittingapparatus 10 includes a first transmitting antenna 14 a and a secondtransmitting antenna 14 b, which are generically called transmittingantennas 14, and the receiving apparatus 12 includes a first receivingantenna 16 a and a second receiving antenna 16 b, which are genericallycalled receiving antennas 16.

The transmitting apparatus 10 transmits predetermined signals, whereasthe first transmitting antenna 14 a and the second transmitting antenna14 b transmit different signals. The receiving apparatus 12 receives thesignals transmitted from the first transmitting antenna 14 a and thesecond transmitting antenna 14 b by the first receiving antenna 16 a andthe second receiving antenna 16 b. The receiving apparatus 12 separatesreceived signals by adaptive array signal processing and demodulates thesignals transmitted from the first transmitting antenna 14 a and thesecond transmitting antenna 14 b independently. Here, if the channelcharacteristic between the first transmitting antenna 14 a and the firstreceiving antenna 16 a is denoted by h₁₁, that between the firsttransmitting antenna 14 a and the second receiving antenna 16 b by h₁₂,that between the second transmitting antenna 14 b and the firstreceiving antenna 16 a by h₂₁, and that between the second transmittingantenna 14 b and the second receiving antenna 16 b by h₂₂, then thereceiving apparatus 12 operates in such a manner as to activate h₁₁ andh₂₂ only by an adaptive array signal processing and demodulate thesignals transmitted from the first transmitting antenna 14 a and thesecond transmitting antenna 14 b independently.

Now, problems to be solved when a preamble signal of a legacy system,for example, the STS thereof is transmitted from each of the firsttransmitting antenna 14 a and the second transmitting antenna 14 b shownin FIG. 3 will be explained. If the signal transmitted from the firsttransmitting antenna 14 a is S₁(t), the signal transmitted from thesecond transmitting antenna 14 b is S₂(t), and the noise is n₁(t) andn₂(t), then X₁(t), or the signal received by the first receiving antenna16 a, and X₂(t), or the signal received by the second receiving antenna16 b, will be expressed respectively as:X ₁(t)=h ₁₁ S ₁(t)+h ₂₁ S ₂(t)+n ₁(t) X₂(t)=h ₁₂ S ₁(t)+h ₂₂ S ₂(t)+n₂(t)  (2)The signal strength in 16 FFT of signals received by the first receivingantenna 16 a is expressed as follows: $\begin{matrix}\begin{matrix}{{\sum{{X_{1}(t)}}^{2}} = {\sum{{X_{1}(t)}{X_{1}^{*}(t)}}}} \\{= {\sum{\left\{ {{h_{11}{S_{1}(t)}} + {h_{21}{S_{2}(t)}} + {n_{1}(t)}} \right\}\left\{ {{h_{11}^{*}{S_{1}^{*}(t)}} +} \right.}}} \\\left. {{h_{21}^{*}{S_{2}^{*}(t)}} + {n_{1}^{*}(t)}} \right\} \\{= {{h_{11}h_{11}^{*}{\sum{{S_{1}(t)}{S_{1}^{*}(t)}}}} + {h_{21}h_{21}^{*}{\sum{{S_{2}(t)}{S_{2}^{*}(t)}}}} +}} \\{{h_{11}h_{21}^{*}{\sum{{S_{1}(t)}{S_{2}^{*}(t)}}}} + {h_{11}^{*}h_{21}{\sum{{S_{1}^{*}(t)}{S_{2}(t)}}}} +} \\{{h_{11}{\sum{{S_{1}(t)}{n_{1}^{*}(t)}}}} + {h_{21}{\sum{{S_{2}(t)}{n_{1}^{*}(t)}}}} +} \\{{h_{11}^{*}{\sum{{S_{1}^{*}(t)}n_{1}}}} + {h_{21}^{*}{\sum{{S_{2}^{*}(t)}{n_{1}(t)}}}} + {\sum{{n_{1}(t)}{n_{1}^{*}(t)}}}}\end{matrix} & (3)\end{matrix}$

Using the relations ΣS₁ ^(*)(t)s₂(t)=XC, ΣS_(i) ^(*)(t)n_(j)(t)=0 and|n_(j)(t)²≈0, the strength is expressed by: $\begin{matrix}\begin{matrix}{{\sum{{X_{1}(t)}}^{2}} = {{h_{11}}^{2} + {h_{21}}^{2} + {h_{11}h_{21}^{*}{Xc}^{*}} + {h_{11}^{*}h_{21}{Xc}}}} \\{= {{h_{11}}^{2} + {h_{21}}^{2} + {2{{Re}\left\lbrack {h_{11}h_{21}^{*}{Xc}^{*}} \right\rbrack}}}}\end{matrix} & (4)\end{matrix}$

When the transmitted signal S₁(t) and S₂(t) are equal to each other andin addition h₁₁=−h₂₁, the strength of received signals is zero, so thatthe AGC of the receiving apparatus 12 does not function accurately.Since Xc in the data interval becomes generally so small as can beregarded as zero, the received power in the data interval becomes|h₁₁|²+|h₂₂|² Hence, the difference in received power between the datainterval and the STS interval is 2Re[h₁₁h₂₁*Xc*] as expressed by thethird term on the right-hand side of Equation (4). This indicates thatthe AGC does not function normally if the Xc in the STS interval islarge and consequently there is a large difference in power between theSTS interval and the data interval. Therefore, an STS different from theSTS of legacy system is required for the MIMO system and thecross-correlation value between them is desired to be low.

Next, explained is a problem caused when a preamble signal, such as STS,suitable for a MIMO system described above is added to a front portionof a packet format. If a packet signal in which the preamble signalsuitable for the MIMO system is added is transmitted, the receivingapparatus 12 can receive said packet signal. On the other hand, areceiving apparatus in the legacy system (not shown) also receives saidpacket signal suitable for the MIMO system. However, the preamblesignals at the legacy system which are stored in the receiving apparatusthereof differ from the preamble signal added to the packet signal.Thus, even if a correlation processing is carried out between them,correlation values will not be greater than a predetermined value. As aresult, the receiving apparatus cannot detect the packet signal. If thereceiving apparatus and the transmitting apparatus are integrallystructured to form a communication apparatus, the aforementionedoperation corresponds to the packet signal not being detected by thecommunication apparatus, so that the transmitting apparatus transmitssignals. This means that the carrier sensing is not accurately performedin the communication apparatus, so that the signal collision is likelyto occur.

FIG. 4 illustrates a structure of a transmitting apparatus 10. Thetransmitting apparatus 10 includes a data separating unit 20, a firstmodulation unit 22 a, a second modulation unit 22 b, . . . and an Nthmodulation unit 22 n, which are generically referred to as modulationunits 22, a first radio unit 24 a, a second radio unit 24 b, . . . andan Nth radio unit 24 n, which are generically referred to as radio units24, a control unit 26, and a first transmitting antenna 14 a, a secondtransmitting antenna 14 b, . . . and an Nth transmitting antenna 14 n,which are generically referred to as transmitting antennas 14. The firstmodulation unit 22 a includes an error correcting unit 28, an interleaveunit 30, a preamble adding unit 32, an IFFT unit 34, a GI unit 36 and aquadrature modulation unit 38. The first radio unit 24 a includes afrequency conversion unit 40 and an amplification unit 42.

The data separating unit 20 separates data to be transmitted, based onthe number of antennas. The error correcting unit 28 performs a codingfor error correction on data. The coding to be employed here is aconvolutional coding, and the coding rate is to be selected fromprescribed values. The interleave unit 30 interleaves data after theconvolutional coding. The preamble adding unit 32 adds a preamble signalto the front portion of a packet signal. Here, the preamble signals thatthe preamble adding unit 32 add are specified for a plurality of kinds.And any of such preamble signals of a plurality of kinds is selectedbased on an instruction from the control unit 26, details of which willbe described later.

The IFFT unit 34 performs IFFT (Inverse Fast Fourier Transform) in unitsof FFT point, thereby converting a frequency-domain signal using aplurality of subcarriers into a signal in time domain. The GI unit 36adds a guard interval to time-domain data. As illustrated in FIG. 2, theguard intervals to be added to the preamble signal and the data signalare different from each other. The quadrature modulation unit 38 carriesout quadrature modulation. The frequency conversion unit 40 performs afrequency conversion by transforming a quadrature-modulated signal intoa radio-frequency signal. The amplification unit 42 is a power amplifierfor amplifying radio-frequency signals. Finally, signals are transmittedin parallel from a plurality of transmitting antennas 14. It is to benoted that in the present embodiment the transmitting antennas 14 arenon-directional and the transmitting apparatus 10 does not performadaptive array signal processing. The control unit 26 controls thetiming and other functions of the transmitting apparatus 10, and selectsa preamble signal which is to be added by the preamble adding unit 32.

In terms of hardware, the above-described structure can be realized by aCPU, a memory and other LSIs of an arbitrary computer. In terms ofsoftware, it is realized by memory-loaded programs which have a reservedmanagement function or the like, but drawn and described herein arefunction blocks that are realized in cooperation with those. Thus, it isunderstood by those skilled in the art that these function blocks can berealized in a variety of forms such as by hardware only, software onlyor the combination thereof.

FIG. 5 illustrates a structure of a control unit 26. The control unit 26includes a selector 110, a monitoring unit 112, a channelcharacteristics acquiring unit 114 and a storage unit 116.

The storage unit 116 stores preamble signals defined in a legacy systemand those defined in a MIMO system. As described earlier, the legacysystem transmits signals using a plurality of subcarriers whereas theMIMO system transmits signals in parallel from a plurality oftransmitting antennas 14 using the same subcarrier number as that of thelegacy system. The preamble signal defined in the MIMO system isspecified in a plurality of kinds in accordance with the number oftransmitting antennas 14 which are to transmit signals. The descriptionwill be given later of the preamble signal specified in the plurality ofkinds. The preamble signal of MIMO system is also defined in such amanner as to contain STSs and LTSs similar to the preamble signal oflegacy system shown in FIG. 2. Here, the signal pattern differs betweenSTS and LTS.

The monitoring unit 112 monitors the presence of a communicationapparatus which is not compatible with the MIMO system but is compatiblewith the legacy system. Here, assume that the transmitting apparatus 10and a receiving apparatus, not shown, constitute integrally acommunication apparatus, for example, a base station apparatuscorresponding to the MIMO system. Among the received signals, thereceiving apparatus searches for signals received from a communicationapparatus of legacy system. That is, it is determined whether or not thepacket format of a received packet format corresponds to the packetformat of legacy system shown in FIG. 2. If the monitoring unit 112 hasnot detected any packet signal defined in the legacy system over apredetermined period of time, it is judged that the communicationapparatus associated with the legacy system does not exist. If, on theother hand, the monitoring unit 112 has detected any packet signaldefined in the legacy system over a predetermined period of time, it isjudged that the communication apparatus associated with the legacysystem exists.

The channel characteristics acquiring unit 114 derives thecharacteristics of a radio channel between the transmitting apparatus 10and a receiving apparatus 12. The characteristics of a radio channel aremeasured by a predetermined method. One method is such that thecharacteristics of a radio channel are measured by the receivingapparatus 12 of FIG. 3. And another method is such that they aremeasured by a communication apparatus including the transmittingapparatus 10. The former corresponds to the characteristics of a radiochannel from the transmitting apparatus 10 toward the receivingapparatus 12 whereas the latter corresponds to the characteristics of aradio channel from the receiving apparatus 12 toward the transmittingapparatus 10. In the former case, it is assumed that the communicationapparatus including the receiving apparatus 12 conveys the measurementresult to the communication apparatus including the transmittingapparatus 10. Here, it is assumed that the characteristics of a radiochannel include received power, delay profile, delay spread, error rateand so forth.

The selector 110 selects a packet format based on the monitoring resultobtained by the monitoring unit 112. Here, the packet format is definedin two kinds. FIGS. 6A and 6B illustrate packet formats selected by theselector 110. FIG. 6A shows a packet format in which a preamble signalcompatible with a MIMO system is placed in the front portion thereof(hereinafter this packet format will be referred to as “dedicatedformat”). Here, assume that signals are transmitted from the firsttransmitting antennas 14 a and the second transmitting antennas 14 bfrom among the transmitting antennas 14 and the packet format of asignal transmitted from the first transmitting antenna 14 a is shown inthe upper section of FIG. 6A and the packet format thereof transmittedfrom the second transmitting antenna 14 b is shown in the lower sectionof FIG. 6A. “STS1” and “LTS1” are transmitted as preamble signals fromthe first transmitting antenna 14 a, and “STSa” and “LTSa” aretransmitted as preamble signals from the second transmitting antenna 14b. Here, “STS1” and STSa” as well as “LTS1” and “LTSa” are signalsdifferent from each other, respectively, and the detail thereof will bedescribed later.

FIG. 6B shows a packet format in which a preamble signal compatible withthe legacy system is further placed before the preamble signalcompatible with the MIMO system (hereinafter this packet format will bereferred to as “mixed format”). Here, the preamble signals STS and LTScompatible with the legacy system will be denoted as “legacy STS” and“legacy LTS”, respectively. The patterns of the legacy STS are asdescribed earlier with reference to FIG. 2. A portion corresponding tothe preamble signals of the MIMO system is the same as that shown inFIG. 6A. “Signal” is placed between the preamble signals compatible withthe legacy system and those compatible with the MIMO system. The“signal” contains information indicating that the preamble signalscompatible with the MIMO system are assigned. Thus, even if thecommunication apparatus of a legacy system receives this packet, it maydiscard this packet signal from the content of the “signal”. Theinformation indicating such the preamble signals are assigned may be thelength of a packet signal. In other words, it suffices if whether acertain signal continues for a certain length of time or not can bedecided.

Since the dedicated format has less redundant signal component, thepacket utilization efficiency can be improved. On the other hand, thepacket signal compatible with the legacy system is added in the mixedformat, so that the mixed format is detected by a communication systemcompatible with the legacy system. If the monitoring unit 112 has notdetected any communication apparatus compatible with the legacy system,the selector 110 selects the dedicated format. And if the monitoringunit 112 has detected the communication apparatus compatible with thelegacy system, the selector 110 selects the mixed format.

The selector 110 selects the allocation of LTS, based on thecharacteristics of a radio channel detected by the channelcharacteristics acquiring unit 114. FIGS. 7A and 7B illustrate theformat of LTS selected by the selector 110. Though in FIGS. 7A and 7Bthe format is described using the dedicated format only, the format usedmay be the mixed format. In that case when the mixed format is used, thepreamble signals of the MIMO system will be shown instead. FIG. 7A showsa case when LTSs are respectively transmitted at the same timing(hereinafter such a format will be referred to as “contiguous format”).“LTS1” is transmitted from the first transmitting antenna 14 a and“LTSa” is transmitted from the second transmitting antenna 14 b. FIG. 7Bshows a case when LTSs are respectively transmitted from a plurality ofantennas 14 at different timings (hereinafter such a format will bereferred to as “separate format”). As shown in FIG. 7B, the timing atwhich “LTS1” is transmitted deviates from the timing at which “LTSa” istransmitted.

Since the contiguous format has less redundant signal component, thepacket utilization efficiency can be improved. On the other hand, theseparate format is such that “LTS1” and “LTSa” are transmitted atdifferent timings and the intersignal interference is reduced. Thus, theestimation of channel characteristics as well as the estimation ofresponse vectors and weight vectors by the receiving apparatus 12,described later, will be accurately done, so that the communicationquality improves. If the characteristic of a radio channel acquired bythe channel characteristics acquiring unit 114, for example, the errorrate has not deteriorated more than a threshold value, then the selector110 selects the contiguous format. And if the error rate has moredeteriorated than the threshold value, the selector 110 selects theseparate format.

FIG. 8 is a table showing a relationship, used when the selection isdone at the selector 110, between the number of transmitting antennas 14and the patterns of STSs transmitted by the transmitting antennas.Though the description concerning LTSs is omitted here, the selectionwill be done in the same manner as with STSs. The table shows thenumbers of transmitting antennas 14 vertically. The table also showshorizontally the transmitting antennas 14 to be used and the STSscorresponding thereto in relation to the numbers of antennas 14. Inother words, when the number of transmitting antennas 14 is “1”, alegacy STS is transmitted from the first transmitting antenna 14 a. Ifthe number of transmitting antennas 14 becomes one when using thededicated format, the selector 110 may transmit “STS1” defined in theMIMO system. As a result of this, the switching otherwise done to thepreamble signal compatible with the legacy system can be skipped.

When the number of transmitting antennas 14 is “2”, “STS1” istransmitted from the first transmitting antenna 14 a and “STSa” istransmitted from the second transmitting antenna 14 b. When the numberof transmitting antennas 14 is “3”, “STS1” is transmitted from the firsttransmitting antenna 14 a, “STS2” is transmitted from the secondtransmitting antenna 14 b and “STSb” is transmitted from the thirdtransmitting antenna 14 c. Here, in order to solve the aforementionedproblem, “STS1”, “STSa”, “STS2” and “STSb” are defined in a manner suchthat the cross-correlation values thereof become small.

The transmitting apparatus further includes a function by which tonotify the receiving apparatus 12 about the number of transmittingantennas 14 that are transmitting the signals, by the pattern differencebetween “STSa” transmitted from the second transmitting antenna 14 bwhen the number of antennas 14 is “2” and “STSb” transmitted from thetransmitting antenna 14 c when the number of transmitting antennas 14 is“3”. As a result, these STSs differ to the degree that “STSa” and “STSb”are identifiable from the signals received by the receiving apparatus12. In other words, the value of cross-correlation between “STSa” and“STSb” is so defined as to be small.

The number of transmitting antennas 14 is determined by the control unit26. The control unit 26 determines the number of transmitting antennas14 in accordance with the characteristics of a radio channel acquired bythe channel characteristics acquiring unit 114. That is, if thecharacteristics of a radio channel are found favorable, the number oftransmitting antennas 14 is increased. The control unit 26 may determinethe number of transmitting antennas 14 based on the information capacityto be transmitted. For instance, if the information capacity to betransmitted is large, the number of transmitting antenna 14 isincreased.

FIG. 9 illustrates a structure of a receiving apparatus 12. Thereceiving apparatus 12 includes a first receiving antenna 16 a, a secondreceiving antenna 16 b, . . . and an Nth receiving antenna 16 n, whichare generically referred to as receiving antennas 16, a first radio unit50 a, a second radio unit 50 b, . . . and an Nth radio unit 50 n, whichare generically referred to as radio units 50, a first processing unit52 a, a second processing unit 52 b, . . . and an Nth processing unit 52n, which are generically referred to as processing units 52, a firstdemodulation unit 54 a, a second demodulation unit 54 b, . . . and anNth demodulation unit 54 n, which are generically referred to asdemodulation units 54, a data combining unit 56, and a control unit 58.Signals used include a first radio received signal 200 a, a second radioreceived signal 200 b, . . . and an Nth radio received signal 200 n,which are generically referred to as radio received signals 200, a firstbaseband received signal 202 a, a second baseband received signal 202 b,. . . and an Nth baseband received signal 202 n, which are genericallyreferred to as baseband received signals 202, and a first compositesignal 204 a, a second composite signal 204 b, . . . and an Nthcomposite signal 204 n, which are generically referred to as compositesignals 204.

The radio units 50 carry out frequency conversion processing betweenradio received signals 200 of radio frequency and baseband receivedsignals 202 of baseband, amplification processing, A-D conversionprocessing and the like. It is assumed here that the radio frequency ofthe radio received signals 200 is in the 5 GHz band. The radio units 50further perform correlation processing for timing detection. Theprocessing units 52 perform adaptive array signal processing on thebaseband received signals 202 and output composite signals 204corresponding to a plurality of transmitted signals. The demodulationunits 54 demodulate the composite signals 204. The demodulation units 54further perform the removal of guard intervals, FFT, deinterleave anddecoding. The data combining unit 56 combines the signals outputtedrespectively from the demodulation units 54 in correspondence to thedata separating unit 20 as shown in FIG. 4. The control unit 58 controlsthe timing and other functions of the receiving apparatus 12.

FIG. 10 illustrates a structure of a first radio unit 50 a. The firstradio unit 50 a includes an LNA 60, a frequency conversion unit 62, aquadrature detector 64, an AGC unit 66, an A-D conversion unit 68 and acorrelator 70.

The LNA 60 amplifies a first radio received signal 200 a. The frequencyconversion unit 62 carries out a frequency conversion between a radiofrequency in the 5 GHz band and an intermediate frequency for a signalto be processed. The AGC unit 66 controls gain automatically so as toturn the amplitude of a signal into an amplitude within the dynamicrange of the A-D conversion unit 68. It is to be noted that, in theinitial setting of the AGC unit 66, the STS in a received signal is usedand control is performed in a manner such that the strength of the STSapproaches a prescribed value. The A-D conversion unit 68 converts ananalog signal into a digital signal. The quadrature detector 64 performsa quadrature detection of an intermediate-frequency signal and generatesa baseband digital signal so as to output it as a first basebandreceived signal 202 a. The baseband signal, which is generally composedof in-phase components and quadrature components, shall be representedby two signal lines. For the sake of clarity in the figure, the basebandsignal is presented here by a single signal line, and the same will beapplied hereinafter.

In order to detect an STS from the first baseband received signal 202 a,the correlator 70 performs a correlation processing on a first basebandreceived signal 202 a and an STS stored in advance and outputs acorrelation value. In the MIMO system the STS is set for each of thetransmitting antennas 14, so that the correlators 70 perform correlationprocessing respectively for a plurality of STSs and output a pluralityof correlation values. The correlation values are inputted to thecontrol unit 58 shown in FIG. 9 via signal lines not shown. The controlunit 58 determines the start of receiving a packet signal based on theplurality of correlation values inputted from the plurality ofcorrelators 70 and sends the decision to the processing units 52, thedemodulation units 54 and others. Also, in order to demodulate aplurality of signals, the control unit 58 determines the assignment ofprocessing units 52 and demodulation units 54 for the respective signalsand sends the decision to the processing units 52 and the demodulationunits 54.

FIG. 11 illustrates a structure of a correlator 70. The correlator 70includes a correlator for legacy STS 330, a correlator for STSa 332, acorrelator for STSb 334 and a selector 336.

The correlator for STSa 332, which stores beforehand signal series whereSTSa has been converted to signal series in the time domain, calculatesa value of correlation between the stored signal series and the receivedsignal series (hereinafter referred to as “correlation value for 2antennas”). The correlator for STSb 334, which stores beforehand signalseries where STSb has been converted into signal series in the timedomain, calculates a value of correlation between the stored signalseries and the received signal series (hereinafter referred to as“correlation value for 3 antennas”).

The correlator for legacy STS 330 stores in advance the signal serieswhere the aforementioned legacy STS has been converted into the timedomain or where subcarrier signals for part of the legacy STS have beenconverted into the time domain. The correlator for legacy STS 330calculates a value of correlation between the stored signal series andthe received signal series (hereinafter referred to as “correlationvalue for 1 antenna”). The signal series stored in the correlator forlegacy STS 330 may be those corresponding to the STS compatible with theMIMO system, for instance, STS1 shown in FIG. 8.

The selector 336 compares a correlation value for 2 antennas, acorrelation value for 3 antennas and a correlation value for 1 antennaand selects a largest correlation value therefrom. An estimation unit,which is not shown here, determines the number of transmitting antennas14 transmitting data, based on the selected correlation value. That is,if the correlation value for 2 antennas is the largest, the number oftransmitting antennas 14 will be determined to be “2”. And if thecorrelation value for 3 antennas is the largest, the number oftransmitting antennas 14 will be determined to be “3”. And if thecorrelation value for 1 antenna is the largest, the number oftransmitting antennas 14 will be determined to be “1”.

FIG. 12 illustrates a structure of a first processing unit 52 a. Thefirst processing unit 52 a includes a synthesizing unit 80, a receivedresponse vector computing unit 82 and a reference signal storage unit84. The synthesizing unit 80 includes a first multiplier 86 a, a secondmultiplier 86 b, . . . and an Nth multiplier 86 n, which are genericallyreferred to as multipliers 86, and an adder 88. Also, signals usedinclude a first received weight signal 206 a, a second received weightsignal 206 b, . . . and an Nth received weight signal 206 n, which aregenerically referred to as received weight signals 206, and a referencesignal 208.

The reference signal storage unit 84 stores LTSs and other signals. Itis assumed herein that LTS is also selected in accordance with STSselected by the correlator for legacy STS 330.

The received response vector computing unit 82 computes received weightsignals 206, as receiving response characteristics of received signalsfor transmitted signals, from baseband received signals 202 andreference signal 208. Although the method for computing the receivedweight signals 206 may be arbitrary, one example, as shown below, isbased on a correlation processing. It is to be noted that the receivedweight signals 206 and the reference signal 208 are inputted not onlyfrom within the first processing unit 52 a but also from the secondprocessing unit 52 b or the like via signal lines not shown. If thefirst baseband received signal 202 a is denoted by x₁(t), a secondbaseband received signal 202 b by x₂(t), the reference signal 208corresponding to the first transmitting antenna 14 a by S₁(t) and thereference signal 208 corresponding to the second transmitting antenna 14b by S₂(t), then x₁(t) and X₂(t) will be expressed by the followingEquation (5):x ₁(t)=h ₁₁ S ₁(t)+h ₂₁ S ₂(t) x₂(t)=h ₁₂ S ₁(t)+h ₂₂ S ₂(t)  (5)The noise is ignored here. A first correlation matrix R₁, with E as anensemble average, is expressed by the following Equation (6):$\begin{matrix}{R_{1} = \begin{bmatrix}{E\left\lbrack {x_{1}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {x_{1}S_{2}^{*}} \right\rbrack} \\{E\left\lbrack {x_{2}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {x_{2}S_{2}^{*}} \right\rbrack}\end{bmatrix}} & (6)\end{matrix}$

A second correlation matrix R₂ for computing the correlation among thereference signals 208 is given by the following Equation (7):$\begin{matrix}{R_{2} = \begin{bmatrix}{E\left\lbrack {S_{1}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {S_{1}S_{2}^{*}} \right\rbrack} \\{E\left\lbrack {S_{2}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {S_{2}S_{2}^{*}} \right\rbrack}\end{bmatrix}} & (7)\end{matrix}$

Finally, the first correlation matrix R₁ is multiplied by the inversematrix of the second correlation matrix R₂ so as to obtain a receivedresponse vector 206, which is expressed by the following Equation (8):$\begin{matrix}{\begin{bmatrix}h_{11} & h_{21} \\h_{12} & h_{22}\end{bmatrix} = {R_{1}R_{2}^{- 1}}} & (8)\end{matrix}$

The received weight signals 206 may be derived using an adaptivealgorithm such as the LMS.

The multipliers 86 weight the baseband received signals 202 with thereceived weight signals 206 respectively, and the adder 88 adds up theoutputs of the multipliers 86 so as to output the composite signal 204.

FIG. 13 is a flowchart showing a procedure for transmission processingin a transmitting apparatus 10. The monitoring unit 112 monitors whetherthere exists any communication apparatus compatible with the legacysystem or not. If the communication apparatus compatible with a legacysystem exits (Y of S10), the selector 110 selects the mixed format(S12). If, on the other hand, the communication apparatus compatiblewith a legacy system does not exist (N of S10), the selector 110 selectsthe dedicated format (S14). Furthermore, the selector 110 selects, fromthe storage unit 116, STS and LTS corresponding to the number oftransmitting antennas 14 (S16) and assigns them within the selectedformat. And the transmitting apparatus 10 transmits the packet signals(S18).

FIG. 14 is a flowchart showing another procedure for transmissionprocessing in the transmitting apparatus 10. The channel characteristicsacquiring unit 114 acquires the characteristics of a radio channel, forexample, the error rate. If the characteristic of a radio channel issatisfactory (Y of S50), that is, if the error rate is smaller than athreshold value, then the selector 110 selects the contiguous format(S52). If, on the other hand, the characteristic of a radio channel isnot satisfactory (N of S50), the selector 110 selects the separateformat (S54). Furthermore, the selector 110 selects, from the storageunit 116, STS and LTS corresponding to the number of transmittingantennas 14 (S56) and assigns them within the selected format. And thetransmitting apparatus 10 transmits the packet signals (S58).

According to the present embodiment, the preamble signal in the legacysystem is added to the front portion of a packet signal, so that thepacket signal can be received by a communication apparatus compatiblewith the legacy system. The compatibility with the legacy system can beretained. The presence of packet signal can be made known to thecommunication apparatus of a legacy system. Since the transmission ofsignals by the communication apparatus of a legacy system is prevented,the probability of signal collision can be lowered. Since the presenceor absence of the preamble signal of the legacy system is switched, theimprovement in the compatibility with the legacy system and the packetutilization efficiency can be selected. The switching between thepresence and the absence of a preamble signal of the legacy system isdone based on whether any legacy system exits or not, thereby giving noadverse effect on other communication apparatus.

Furthermore, the pattern of preamble signal is changed in accordancewith the number of antennas, so that the communication quality can beimproved. Even when the number of antennas becomes one, the preamblesignal corresponding to a single one of a plurality of antennas is used,thus eliminating the trouble of switching to the legacy system. A signalis inserted after the preamble signal of a legacy system. Thus, thecontent of such a subsequent signal can be conveyed to a communicationapparatus of the legacy system. The structure of preamble signals to betransmitted from a plurality of antennas is changed, so that selectioncan be made as to the transmission quality of signals and the packetutilization efficiency. The structure of preamble signals to betransmitted from a plurality of antennas is varied based on the qualityof a radio channel, so that the structure of a preamble suitable for theradio channel can be selected.

The present invention has been described based on the embodiments whichare only exemplary. It is understood by those skilled in the art thatthere exist other various modifications to the combination of eachcomponent and process described above and that such modifications areencompassed by the scope of the present invention.

In the above embodiments, the wireless LAN that conforms to theIEEE802.11a standard is used as an example of the legacy system, but isnot limited thereto and other communication systems may also be used.Though in the above embodiments the communication system 100 isillustrated as a MIMO system, but it is not limited thereto and othercommunication systems may be used. The multi-carrier signal may not betransmitted. The present embodiments can be applied to various types ofcommunication systems 100. That is, it is preferable that the legacysystem and the communication system 100 have some sort of compatibilitysuch as the identical radio frequency.

Although the present invention has been described by way of exemplaryembodiments and modifications, it should be understood that many otherchanges and substitutions may further be made by those skilled in theart without departing from the scope of the present invention which isdefined by the appended claims.

1. A transmitting apparatus, comprising: a storage unit which stores afirst known signal defined in a first wireless communication system anda second known signal defined in a second wireless communication systemwhich differs from the first wireless communication system; a selectorwhich selects either a first packet format in which the second knownsignal is placed in a front portion thereof or a second packet format inwhich the first known signal is further placed before the second knownsignal; and a transmitter which transmits signals in the packet formatselected by said selector.
 2. A transmitting apparatus, comprising: astorage unit which stores a first known signal defined in a firstwireless communication system which is to transmitinverse-fast-Fourier-transformed signals and a second known signaldefined in a second wireless communication system which is to transmitinverse-fast-Fourier-transformed signals from a plurality of antennas inparallel; a selector which selects either a first packet format in whichthe second known signal is placed in a front portion thereof, or asecond packet format in which the first known signal is further placedbefore the second known signal; and a transmitter which transmits theinverse-Fourier-transformed signals in the packet format selected bysaid selector.
 3. A transmitting apparatus, comprising: a storage unitwhich stores a first known signal defined in a first wirelesscommunication system which is to transmit signals using a plurality ofcarriers and a second known signal defined in a second wirelesscommunication system which is to transmit signals from a plurality ofantennas in parallel, using the same number of carriers to transmit thesignals as in the first wireless communication system; a selector whichselects either a first packet format in which the second known signal isplaced in a front portion thereof, or a second packet format in whichthe first known signal is further placed before the second known signal;and a transmitter which transmits signals in the packet format selectedby said selector.
 4. A transmitting apparatus according to claim 2,wherein the second known signal stored in said storage unit is specifiedin a plurality of kinds in accordance with the number of antennas whichare to transmit signals in the second wireless communication system. 5.A transmitting apparatus according to claim 3, wherein the second knownsignal stored in said storage unit is specified in a plurality of kindsin accordance with the number of antennas which are to transmit signalsin the second wireless communication system.
 6. A transmitting apparatusaccording to claim 4, wherein if the packet format in which the secondknown signal is placed in a front portion thereof is selected and thenumber of antennas to transmit signals is one, said selector assigns oneof the second known signals in which the plurality of kinds arespecified.
 7. A transmitting apparatus according to claim 2, whereinwhen the second packet format in which the first known signal is furtherplaced before the second known signal is selected, said selector assignsinformation indicating that the second known signal is placed, betweenthe first known signal and the second known signal.
 8. A transmittingapparatus according to claim 3, wherein when the second packet format inwhich the first known signal is further placed before the second knownsignal is selected, said selector places information indicating that thesecond known signal is placed, between the first known signal and thesecond known signal.
 9. A transmitting apparatus according to claim 2,further comprising a monitoring unit which monitors the presence of acommunication apparatus which is not compatible with the second wirelesscommunication system and is compatible with the first wirelesscommunication system, wherein said selector selects a packet formatbased on a monitoring result obtained from said monitoring unit.
 10. Atransmitting apparatus according to claim 3, further comprising amonitoring unit which monitors the presence of a communication apparatuswhich is not compatible with the second wireless communication systemand is compatible with the first wireless communication system, whereinsaid selector selects a packet format based on a monitoring resultobtained from said monitoring unit.
 11. A transmitting apparatusaccording to claim 2, wherein the second signal stored in said storageunit has a plurality of portions whose signal patterns differ from eachother, and wherein said selector selects either a first assignment ofthe second known signal in which at least one of the plurality ofportions are transmitted respectively at the same timing from aplurality of antennas or a second assignment of the second known signalin which at least one of the plurality of portions are transmittedrespectively at different timings from the plurality of antennas.
 12. Atransmitting apparatus according to claim 3, wherein the second signalstored in said storage unit has a plurality of portions whose signalpatterns differ from each other, and wherein said selector selectseither a first assignment of the second known signal in which at leastone of the plurality of portions are transmitted respectively at thesame timing from a plurality of antennas or a second assignment of thesecond known signal in which at least one of the plurality of portionsare transmitted respectively at different timings from the plurality ofantennas.
 13. A transmitting apparatus according to claim 11, furthercomprising a derivation unit which derives characteristics of a radiochannel through which signals are to be transmitted, wherein saidselector selects either the first assignment or second assignment of thesecond known signal, based on the characteristics of a radio channelderived by said derivation unit.
 14. A transmitting apparatus accordingto claim 2, wherein said selector selects either a first packet formatin which the second known signal is placed at the same timing in aplurality of antennas or a second packet format in which the secondknown signal is placed at different timings in the plurality ofantennas.
 15. A transmitting apparatus according to claim 3, whereinsaid selector selects either a first packet format in which the secondknown signal is placed at the same timing in a plurality of antennas ora second packet format in which the second known signal is placed atdifferent timings in the plurality of antennas.
 16. A transmittingapparatus, comprising: a transmitter which transmits signals defined ina predetermined packet format, in parallel from a plurality of antennas;a storage unit which stores a known signal to be placed in a frontportion of a packet format; and a selector which selects, at the time ofplacing a known signal in the front portion of a packet format, a firstassignment in which the known signal is transmitted at the same timingfrom the plurality of antennas or a second assignment in which the knownsignal is transmitted at different timings from the plurality ofantennas.
 17. A transmitting apparatus, comprising: a transmitter whichtransmits signals defined in a predetermined packet format, in parallelfrom a plurality of antennas; a storage unit which stores a plurality ofkinds of known signals to be placed in a front portion of a packetformat; a first selector which selects a known signal from among theplurality of kinds of known signals stored in said storage unit; and asecond selector which selects, at the time of placing the known signalselected by said first selector in the front portion of a packet format,either a first packet format in which the selected known signal isplaced at the same timing in the plurality of antennas or a secondpacket format in which the selected known signal is placed at differenttimings in the plurality of antennas.
 18. A transmitting apparatus,comprising: a transmitter which transmits signals defined in apredetermined packet format, in parallel from a plurality of antennas; astorage unit which stores a plurality of kinds of known signals to beplaced in a front portion of a packet format; and a selector whichselects, at the time of placing a known signal in a front portion of apacket format, either a first packet format in which known signals ofdifferent patterns are placed in the plurality of antennas or a secondpacket format in which known signals whose transmission timings areshifted are placed in the plurality of antennas.
 19. A transmittingapparatus according to claim 16, further comprising a derivation unitwhich derives characteristics of a radio channel through which signalsare to be transmitted, wherein said selector selects an assignment ofthe known signals based on the characteristics of a radio channelderived by said derivation unit.
 20. A transmitting apparatus accordingto claim 17, further comprising a derivation unit which derivescharacteristics of a radio channel through which signals are to betransmitted, wherein said first or second selector selects an assignmentof the known signals based on the characteristics of a radio channelderived by said derivation unit.
 21. A transmitting apparatus accordingto claim 18, further comprising a derivation unit which derivescharacteristics of a radio channel through which signals are to betransmitted, wherein said selector selects an assignment of the knownsignals based on the characteristics of a radio channel derived by saidderivation unit.
 22. A transmitting method characterized in that a firstknown signal defined in a first wireless communication system which isto transmit signals using a plurality carriers is specified, a secondknown signal defined in a second wireless communication system which isto transmit signals in parallel from a plurality of antennas using thesame number of carries as the number of carries through which totransmit the signals are specified, and the signals are transmitted byselecting either a first packet format in which the second known signalis placed in a front portion or a second packet format in which thefirst known signal is further placed before the second known signal. 23.A transmitting method characterized in that, for a known signal which isto be placed in a front portion of a packet format of a signal to betransmitted in parallel from a plurality of antennas, either a firstassignment in which the known signal is transmitted at the same timingfrom the plurality of antennas or a second assignment in which the knownsignal is transmitted at different timings from the plurality ofantennas is selected.